Oxidation of #alpha#-hydroxyacids by N-bromophthalimide-Dependence of mechanism on pH of the medium

摘要

The oxidation of #alpha#-hydroxyacids, namely mandelic acid, lactic acid, malic acid, benzilic acid and atrolatic acid and N-bromophthalimide to give the corresponding carbonyl compounds, has been carried out. It was of interest to determine whether the alcoholic OH or the carboxylic OH is involved in the oxidative decarboxylation of #alpha#-hydroxy acids which have bifunctional groups. A study of the dependence of rate on the pH of the medium was ecpected to differentiate between these two routes. Further, the #alpha#-hydroxyacids may react as simple alcohols and undergo oxidation involving a C-H bond cleavage to give keto acids which may subsequently undergo decarboxylation. A comparative study of mandelic acid and mandelic acid-#alpha#_d indicates the absence of primary kinetic isotope effect, ruling out the above possibility. It is likely that acyl hypobromite is a precursor for oxidation at higher pH and the involvement of alkyl hypobromite cannot be ruled out at lower pH. The NBP and NBS oxidations of #alpha#-hydroxyacids are found to be well correlated.